1. Field of the Invention
This invention relates to a sensing device for measuring the value of an unknown capacitance, and more particularly, to a level sensing device using a capacitance whose value is dependent upon the level of liquid in a tank.
2. Description of the Prior Art
Previously proposed sensing systems, such as U.S. Pat. No. 4,083,248, incorporate the use of a capacitive probe in combination with a pair of threshold detectors. The capacitance is connected to timing circuitry which includes the threshold detectors. Such a level sensing system using more than one threshold detector suffers from the disadvantage of being sensitive to temperature changes which is reflected as an instability in the period of the output wave form. Furthermore, the prior system is relatively expensive because it requires the use of more than one operational amplifier.
It is an object of the invention to provide an improved capacitive level sensing device with only a single operational amplifier so as to be insensitive to temperature changes.
It is another object of the invention to provide a capacitive level sensing device which utilizes a small number of parts resulting in a lower cost capacitance transmitter with good stability over a wide temperature range.